Metal oxide or metal oxynitride thin film transistors have many advantages such as a higher carrier density and higher mobility, and have found a wide range of applications in display field. Moreover, the metal oxide or metal oxynitride thin film transistors have the advantages of lower manufacturing costs, higher transmittance, and higher bad gap. In recent years, the metal oxide or metal oxynitride thin film transistors are made smaller, and the display panel having such thin film transistors can achieve a higher resolution and a better display effect.